The Stable Topology of the Planetary Systems of two 2:1 Resonant Companions:Application to HD 82943

0.56 MB·English

by  Ji Jianghui

The stable topology of the planetary systems
of two 2:1 resonant companions:application to
HD 82943
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JI J.a,b, , Kinoshita H.c, LIU L.d,b, Nakai H.c, LI G.a,b
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2 Purple Mountain Observatory, CAS, Nanjing 210008, China
n bNational Astronomical Observatory, CAS, Beijing 100012, China
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J National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan

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Department of Astronomy, Nanjing University, Nanjing 210093, China
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Abstract
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0 We have numerically explored the stable planetary geometry for the multiple sys-
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tems involved ina2:1 mean motion resonance, andherein wemainly concentrate on
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/ the HD 82943 system by employing two sets of the orbital parameters (Mayor et al.
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p 2004; Ji et al. 2004). In the simulations, we find that all stable orbits are related to
- the 2:1 resonance that can help to remain the semi-major axes for two companions
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r almost unaltered over the secular evolution for 10 yr. In addition, we also show
st that there exist three possible stable configurations:(1) Type I, only θ1 ≈ 0◦, (2)
:a Type II, θ1 ≈ θ2 ≈ θ3 ≈ 0◦ (aligned case), and (3) Type III, θ1 ≈ 180◦, θ2 ≈ 0◦,
v θ3 ≈ 180◦ (antialigned case), here the lowest eccentricity-type mean motion res-
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X onant arguments are θ1 = λ1 − 2λ2 + ̟1 and θ2 = λ1 − 2λ2 + ̟2, the relative
r apsidal longitudes θ3 = ̟1 −̟2 = ∆̟. And we find that other 2:1 resonant sys-
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tems (e.g., GJ 876) may possess one of three stable orbits in their realistic motions.
Furthermore, we show that the assumed terrestrial bodies cannot survive near the
habitable zones for HD 82943 due to strong perturbations induced by two resonant
companions, but such low-mass planets can bedynamically habitable in the GJ 876
system at ∼ 1 AU in the numerical surveys.
Key words: methods:N-body simulations — celestial mechanics — planetary
systems — stars:individual (HD 82943, GJ 876)
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Corresponding author.
Email address: [email protected] (JI J.).
Preprint submitted to Advances in Space Research 2 February 2008
1 Introduction
The discovery of the extrasolar planets should show the diversity of the plan-
etary systems in the universe and also sketch out an innovative world outside
our solar system. Mayor & Queloz (1995) began such great revolution in our
minds when they found the first extrasolar giant Jupiter-51 Peg, and to date
there are more than 100 planetary systems (see also a daily updated web-
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site ; Butler et al. 2003), which most of them were detected with the radial
velocity technique in the surveys of nearby young stars. At present, a dozen
of multiple planetary systems were discovered in recent years and this number
is undoubtedly cumulative as time elapses. Hence, it is necessary to catego-
rize discovered multiple systems according to their statistical characteristics
(such as the distribution of the planetary masses, semi-major axes, eccentric-
ities and metallicity), then to study the correlation between mass ratio and
period ratio (Mazeh & Zucker 2003) and to improve the understanding of the
correlation between the planet occurrence rate and stellar metallicity (San-
tos et al. 2003; Fischer, Valenti, & Marcy 2004). The other key point is to
investigate possible stable configurations for the multiple systems, in which
the observations reveal that most of them are typically characterized by mean
motion resonance (MMR) and (or) apsidal phase-locking between their orbit-
ing companions (Fischer et al. 2003; Ji et al. 2003a), so that one can better
understand the full dynamics of these systems.
2 Stable Planetary Geometry in 2:1 Mean Motion Resonance
Amongst the discovered extrasolar planetary systems, the resonant pairs are
quite common for the multiple systems, e.g., HD 82943 (Gozdziewski & Ma-
ciejewski 2001) and GJ 876 (Lee & Peale 2002; Ji, Li & Liu 2002) in a 2:1
resonance, and 55 Cancri in a 3:1 MMR (Marcy et al. 2002; Ji et al. 2003b).
In addition, as the observation results show that most of the systems can po-
tentially harbor more than one planet, thus we turn to the fascinating topic of
studying the stable planetary geometry locking into a resonance, which such
investigations are expected to be helpful in an understanding of the orbital
evolution of the planets and the dynamical mechanisms of sustaining the sta-
bility of such systems. In the present study, we mainly focus our attention on
the HD 82943 system. In Table 1 are listed the orbital parameters for this sys-
tem, where two sets of the best-fit solutions are respectively given according
to Mayor et al. (2004) (Fit 2) and their original orbital data (Fit 1; as of July
31, 2002), which are adopted in our simulations.
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http://cfa-www.harvard.edu/planets/bibli.html
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Table 1. Orbital parameters of HD 82943 planetary system. The stel-
lar mass Mc, for Fit 1, Mc = 1.05M⊙, while for Fit 2, Mc = 1.15M⊙
Planet Mass(Mjup) P(day) a(AU) e ω(deg) τ(JD)
b (Fit 1) 1.63 444.6(8.8) 1.16 0.41(0.08) 96(7) 2451620.3(12)
c (Fit 1) 0.88 221.6(2.7) 0.73 0.54(0.05) 138(13) 2451630.9(5.9)
b (Fit 2) 1.84 435.1(1.4) 1.18 0.18(0.04) 237(13) 2451758(13)
c (Fit 2) 1.85 219.4(0.2) 0.75 0.38(0.01) 124(3) 2452284(1)
In this paper, we employ N-body codes (Ji et al. 2002) to perform direct nu-
merical simulations by using RKF7(8) and symplectic integrators (Wisdom
& Holman 1991) for this system. In the simulations, we always take the stel-
lar mass and the minimum planetary masses from Table 1. The time stepsize
is usually adopted to be ∼ 1%-2.5% of the orbital period of the innermost
planet, which is sufficiently smaller for the integration. Additionally, the nu-
merical errors were effectively controlled with the accuracy of 10−14 over the
integration timescale.
In our simulations for HD 82943 system, we found three possible stable con-
figurations for a system in a 2:1 MMR: (I) only θ1 librates about 0◦, (II)
θ1 ≈ θ2 ≈ θ3 ≈ 0◦ (aligned case), (III) θ1 ≈ 180◦, θ2 ≈ 0◦, θ3 ≈ 180◦
(antialigned case). The definition of the lowest eccentricity-type 2:1 resonant
arguments are θ1 = λ1 −2λ2 +̟1 and θ2 = λ1 −2λ2 +̟2. And the relative
apsidal longitude is θ3 = ∆̟ = ̟1 − ̟2, where λ1,2 are, respectively, the
mean longitudes of the inner and outer planets, and ̟1,2, respectively, their
periastron longitudes. Obviously, for above three angles, no more than two
are linearly independent. Besides, these stable planetary geometry is related
to the existence of symmetric stable equilibrium solutions, where we find that
other 2:1 resonant systems (e.g., GJ 876) may possess one of the above three
stable orbits in their realistic motions.
As a paradigm, we simply show the dynamical evolution for one of the types
of the stable orbits for HD 82943 and the reader may refer to Ji et al. (2004)
for a detailed study. Figures 1 and 2 exhibit that the long-term orbital evolu-
tion for one set of stable solutions derived from Fit 2, where θ1 and θ3 both
librate about 0◦ (Type II) for 108 yr, meanwhile the semi-major axes and ec-
centricities for two massive planets perform slight vibrations. Still, one may
understand that the 2:1 resonance can help remain the semi-major axes for
two companions almost unaltered over the secular evolution (see Figs. 1 and
2), and the apsidal phase-locking between two orbits can further enhance the
stability for this system, because the eccentricities are simultaneously pre-
served to restrain the planets from frequent close encounters. Our results are
well consistent with previous works, e.g., Hadjidemetriou (2002) numerically
studied the families of periodic orbits for the HD 82943 system under the ro-
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tating framework, and indicated that two kinds of families orbits can survive
for the 2:1 resonant planets. Moreover, Beauge et al. (2003) further showed
the asymmetric stable solutions for the 2:1 resonant system using both ana-
lytical and numerical means. In summary, all the above outcomes imply the
potential planetary configurations that the extrasolar systems involved in 2:1
resonance may hold.
In order to understand the dynamics of this system, in a companying paper
(see Ji et al. 2004), we analytically plot the evolution of the eccentricities and
the relative apsidal longitudes. The semi-analytical contour chart exhibits a
good agreement with the numerical investigations of maintaining the eccen-
tricities with larger θ3-libration amplitudes (ref. to Fig.4b of Ji et al. 2004;
Type II), and the figure also implies that the θ3-libration will be broken up if
the libration amplitudes exceed or approach the critical value of 90◦ near the
separatrix. Bois (2003, private communication) also confirmed that the stable
strip of the alignment for the initial ̟1 −̟2 is much narrower than that of
the antialignment in the calculations for HD 82943.
3 Habitable Zones
The Habitable zones (HZ) are generally convinced to be suitable places for
terrestrial planets that can provide the liquid-water, subtle temperature and
atmosphere environment, and other proper conditions (Kasting et al. 1993),
supporting the development and biological evolution of life on their surfaces.
And herein, we performed numerical surveys to examine the potential habit-
able zones for the systems (e.g. HD 82943 and GJ 876) with a stable geometry
in 2:1 resonance. We generated 100 seed planets that all bear the same masses
as Earth in each run. The distribution of the initial orbital elements for these
postulated planets are as follows : they move on the much less inclined belt
with the relative inclination with regard to the orbital plane of the resonant
companions less than 5◦, with 0.96 AU≤ a ≤ 1.05 AU and 0 ≤ e ≤ 0.1.
Figure 3 summarizes the main results of the simulations for the HD 82943
system - no Earth-mass planet can survive about 1 AU in the final system,
where 95% of the orbits are usually ejected at t ≤ 105 yr due to the gravi-
tational scattering arising from two close massive planets. The scenario of a
typical ejected orbit may replay that the semi-major axis a grows from ∼ 1
AU to tens of AU, meanwhile the eccentricity e undergoes a rapid increase
from ∼ 0.1 to 1. Hence, the assumed Earth-like planet is thrown out from the
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two-planet system in less than 10 yr. As for GJ 876, all tests about 1 AU are
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stable at least for 10 yr. Figure 4 exhibits the typical orbital evolution for
the Earth-like planet, both the semi-major axis and the eccentricity execute
small fluctuations about 1 AU and 0.06, respectively, and the inclination also
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remains less than 2 degrees in the same time span. And there is no sign to
indicate that such regular orbits at∼ 1 AU with low eccentricities will become
chaotic for much longer time even for the age of the star.
In addition, we also carried out extended integrations for the HD 82943 plane-
tary system to investigate whether there may exist the analogous structure of
asteroidal belts that the residual planetesimals are left in the system in later
planetary formation. We let several hundreds of test particles initially located
at the regions of the 2:1, 3:1 and 5:2 MMRs with the inner massive planet
and then performed the simulations for 1 Myr. The results show that most
of the small bodies can be removed at the dynamical time ∼ 104 yr, which
suggests that no survivals can finally stay at above three resonant regions. In
this sense, it is quite similar to Kirkwood gaps for the main belt asteroids in
our solar system. However, there is much prospect of the space observations
with high resolutions (e.g., Spitzer) revealing the evidence of the disk debris
structure in other extrasolar planetary systems in future.
Acknowledgments This work is financially supported by National Natural Sci-
ence Foundation of China (Grant No. 10203005, 10173006, 10233020) and
Foundation of Minor Planets of Purple Mountain Observatory.
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Fig. 1.Onesetof thestable solutions derived fromFit 2(aligned orbits).Long-term
orbital evolution for θ1 and θ3, where θ1 librates (by yellow color) about 0◦ with a
moderate amplitude of ∼ 60◦, θ3 (by green color) about 0◦ with a larger amplitude
of ∼ 70◦ for 108 yr, in association with Type II orbit. The 2:1 resonance for this
system is confirmed by the modulations of the resonant angles, which reveals the
regular motion for two massive planets (see also Figure 2).
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Fig. 2. Long-term orbital evolution for a and e for two planets (the same as Fig-
ure 1, yellow and green colors, respectively, for the inner and outer planets). The
semi-major axes a1 anda2 arealmost unchangedandtheymodulateabout0.75 and
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1.18 AU with relatively smaller amplitude for 10 yr. In the same time, the ampli-
tude of the oscillations for e1 and e2 are not so large and they are just wandering
in the span (0.35, 0.45) and (0, 0.20), respectively. No signs show that this system
will be chaotic for even longer evolution.
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Ejecting Time Scale (yr)
Fig. 3. The numbers of the ejected terrestrial planet vs ejecting timescale. Notice
that 95% of the orbits are ejected at t ≤ 1.0×105 yr.
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Fig. 4. Orbital evolution of an Earth-mass planet placed in GJ 876 at 1 AU. The
results show that such orbits may exist and be stable in the planetary system.
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